Cells as a model to identify potential taste modulators

ABSTRACT

Co-expression of T1R2 and T1R3 results in a taste receptor that responds to sweet taste stimuli, including naturally occurring and artificial sweeteners. Cells such as U2-OS, which express a functional sweet receptor, can be used in cell-based assays to detect cellular responses to tastants.

Each reference cited in this disclosure is incorporated herein in its entirety.

This application incorporates by reference a 13.7 kb text file created on May 5, 2014 and named “056943.01231sequencelisting.txt,” which is the sequence listing for this application.

TECHNICAL FIELD

This disclosure relates generally to cell lines and assays that can be used to identify sweet taste receptor modulators.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-B. Ca⁽²⁺⁾ response to sweet-tasting compounds. FIG. 1A, Dose-response curves of Ca⁽²⁺⁾-release in JUMP-IN™ T-REX™ U2-OS with (diamond) and without (triangle) T1R3/T1R2/Gα15 activation (n=3 experiments). FIG. 1B, Effects of lactisole and U73122 on Ca⁽²⁺⁾ response in JUMP-IN™ T-REX™ U2-OS cells (mean±SD).

FIG. 2. Graphs showing effect of Gα15 protein on U2-OS response to 100 mM sucralose. U2-OS cells were transiently transfected with 3, 6 and 12 gl/40,000 cells of BacMam encoding G protein, gustducin (Gα15) during 24 h (left) or 48 h (right).

FIG. 3. Graph showing effect of natural sugars (glucose, fructose, and sucrose) at 100 mM on U2-OS cells transfected with 0 or 6 μl/40,000 cells of BacMam encoding Gα15.

FIG. 4. Graph showing that sweet receptor activates pERK½ in U2-OS cells upon treatment with glucose at 100 mM. Treatment of U2-OS cells with lactisole (20 mM, left bars) blocked D-glucose-mediated activation of pERK½.

FIG. 5. Graph demonstrating that transfection of U2-OS cells with Gα15 (6 μl/40,000 cells) significantly increases pERK½ activation, which was blocked by PLCβ2 inhibitor U73122 (10 μM).

FIG. 6. Graph showing pERK½ activation upon treatment with D-glucose and maltose in U2-OS cells transfected with Gα15 (6 μl/40,000 cells).

FIG. 7. Graph showing pERK½ activation upon treatment with different sugars in U2-OS cells transfected with Gα15 (6 μl/40,000 cells).

FIG. 8A-B. Transfluor assay. FIG. 8A, Formation of green fluorescent vesicles in Transfluor U2-OS cells transfected with BacMam encoding Gα15. Nuclei are counterstained blue with Hoechst 33342. FIG. 8B, Quantification of T1R2/T1R3 internalization in Transfluor model (means±SD).

FIG. 9A-B. U2-OS cells express endogenous sweet receptor T1R2/T1R3. FIG. 9A, U2-OS cells were fixed and processed for indirect immunofluorescence with antibodies against sweet receptor T1R2 and T1R3. All images were acquired with a 20× objective using ImageXpress Micro automated microscope. Hoechst 33342 staining is pseudo-colored blue, and antibody-specific staining is pseudo-colored green. FIG. 9B. Western blot of T1R2 in U2-OS and NCI-H716 cells. Cells were lysed in RIPA buffer (50 mM Tris-HCl pH 7.5, 150 mM NaCl, 1% NP-40, 5 mM EDTA, 0.1% SDS, and 10 μg/ml proteinase inhibitors). After freeze-thaw cycles, loading buffer was added at final concentration 2×, samples were heated at 95° C. for 10 minutes and spun in a microcentrifuge for 10 min at highest speed at RT. Supernatant was loaded on gel for analysis by immunoblotting. The primary antibodies for T1R2 and GAPDH were from ThermoFisher.

DETAILED DESCRIPTION

Co-expression of T1R2 and T1R3 results in a taste receptor that responds to sweet taste stimuli, including naturally occurring and artificial sweeteners. Sweet ligands bind to the T1R2/T1R3 receptor and activate G-protein pathway transduction, which includes receptor internalization and intracellular calcium mobilization, as well as induction of down-stream targets such as the phosphorylation of ERK½ (Jang et al., “Gut-expressed gustducin and taste receptors regulate secretion of glucagon-like peptide-1,” Proc Natl Acad Sci U S A. 2007 Sep. 18;104(38):15069-74).

Any cell that comprises or can be engineered to comprise a functional sweet taste receptor can be used in the disclosed assay. In some embodiments, the cells express one or more sweet taste receptors. In some embodiments, the cells are genetically modified to express or to overexpress sweet taste receptors. For example, a cell can be genetically modified by introducing an exogenous nucleic acid encoding T1R2 and T1R3 or two separate exogenous nucleic acids encoding T1R2 and T1R3, respectively. Amino acid sequences for these proteins are known; e.g., GenBank Accession No. NP 689418.2 (T1R2) and GenBank Accession No. NP_689414.1 (T1R3).

Cells can be engineered to express T2R14, T1R2/T1R3, an α-gustducin, and/or GLUT4 using methods well known in the art. GLUT4 is described, inter alia, in U.S. Pat. No. 7,799,538 and references cited therein. See U.S. Pat. No. 8,338,115 and references cited therein and Adler et al., Cell 100, 693-702, 2000 for descriptions of α-gustducin.

In some embodiments, cells are genetically modified to overexpress an α-gustducin (e.g., Gα16, Gα15, Gα16gust25, Gα15gust25, Gα16gust44, Gα15gust44, Gα15-i/3-5), which increases the sensitivity of the assays. SEQ ID NO:1 is the amino acid sequence of Gα15. Nucleotide sequences which can be used to express Gα16 (SEQ ID NO:2), Gα16gust25 (SEQ ID NO:3), Gα15 (SEQ ID NO:4), Gα15gust25 (SEQ ID NO:5), Gα16gust44 (SEQ ID NO:6), Gα15gust44 (SEQ ID NO:7), and Gα15-i/3-5 (SEQ ID NO:8) are provided in the sequence listing; however, any nucleotide sequences which encode these a-gustducins can be used to express them.

In some embodiments, U2-OS cells are used, as described below. Human U2-OS cells (ATCC catalog # HTB-96) express a functional sweet receptor (FIGS. 9A-B). U2-OS cells can therefore be used to detect cellular responses to potential sweet tastants by contacting U2-OS cells with a test compound and measuring sweet taste receptor activity. A variety of functional assays can be used, including measurement of changes in intracellular calcium concentration (e.g., Fluorometric Imaging Plate Reader-based Ca²⁺ mobilization assay, FLIPR), pERK½ activation (e.g., high content imaging, HCl), and receptor internalization (e.g., transfluor). A change in sweet taste receptor activity by a sweetener in the presence of a test compound indicates modulation of the sweet taste receptor by the test compound, thereby identifying a sweet taste modulator (e.g., a molecule which itself provides a sweet taste, a molecule which blocks some or all of a bitter taste, a molecule which enhances the sweet taste of another molecule). An increase in sweet taste receptor activity indicates that the test compound is a sweet taste modulator. Sweet taste modulators can be included in various consumables, including foods, beverages, and pharmaceuticals.

Other cells that can be used in the disclosed assay include, but are not limited to, 1A2, ARH-77, RWPE-1, WI-38, EJM, NCI-H1155, L-1236, NCI-H526, JM1, SHP-77, SNU-878, NCI-H2196, C3A, CA46, SNU-466, KS-1, SNU-738, MOLP-2, HDLM-2, Pfeiffer, HCC-15, Alexander cells, L-540, KMS-12-BM, JK-1, NCI-H1092, SW 1990, NCI-H1184, SU-DHL-1, Hep 3B2.1-7, P3HR-1, NCI-H2029, SU-DHL-5, SNU-1, MOLP-8, SUP-M2, MONO-MAC-1, SNU-1040, KYM-1, HEC-59, HCC1569, OCI-LY3, Hs 819.T, DU4475, CI-1, S-117, OVCAR-8, SNU-626, HL-60, SUIT-2, T3M-4, RKO, MOR/CPR, DK-MG, GA-10, OCUM-1, HCT-15, HT, MONO-MAC-6, G-402, Toledo, COV362, SU-DHL-8, Daoy, NCI-H1435, LS513, Hs 839.T, Hs 172.T, BT-483, KMS-21BM, AGS, NCI-H2172, LC-1/sq-SF, SNU-201, NUGC-4, SK-HEP-1, SUP-B15, SNU-5, HT-1197, SUP-T1, AMO-1, KU812, AN3 CA, AML-193, VMRC-RCW, HLE, HuH28, Hs 751.T, NCI-H2110, MEG-01, MV-4-11, Hep G2, KYSE-30, KALS-1, BICR 6, RMUG-S, JHH-6, Ki-JK, IST-MES1, HCC-95, HPB-ALL, HSC-3, 697, LOU-NH91, KARPAS-299, GI-1, COLO 792, SK-N-FI, D341 Med, HGC-27, SR-786, COLO-818, MHH-CALL-2, SF126, NCI-H322, A-253, NCI-H1623, MCF7, HCC-44, FU97, OCI-LY-19, Hs 766T, NCI-H522, RL, HCC1428, RPMI 6666, U-937, NCI-H460, SW 1088, NCI-H1792, NCI-H1693, UACC-257, JHUEM-2, HuT 78, UACC-893, NCI-H929, A-704, OV56, LN-229, OE19, SK-MEL-24, RD-ES, NCI-H211, KCI-MOH1, NCI-H1963, Hs 706.T, ChaGo-K-1, EPLC-272H, OPM-2, KHM-1B, A549, HuGl-N, NCI-HSO8, MHH-CALL-3, SNU-1076, A3/KAW, MEL-HO, TO 175.T, Caki-1, Hs 936.T, SK-LU-1, WM-983B, K-562, EFE-184, SNU-520, NCI-H2291, HCC-1195, ABC-1, KE-39, NH-6, HCC2218, CMK, RS4;11, KYSE-450, OV7, KYSE-510, SK-UT-1, SNU-C1, OE33, P12-ICHIKAWA, DLD-1, COV434, HuNS1, SNU-899, SW480, COLO-678, LU99, KOPN-8, NCI-H2227, SW1463, Hs 675.T, JHH-4, NCI-H1703, HEC-1-A, BDCM, MIA PaCa-2, PC-3, TE-15, PK-45H, MKN-45, HCC-366, CAL-29, HEC-50B, CPC-N, KMRC-20, SW1116, EOL-1, COLO 205, EHEB, YD-38, MC116, SK-N-BE(2), BV-173, NCI-H2347, LU65, RT4, U-87 MG, LK-2, KP-N-YN, HEC-251, NCI-H1651, GP2d, RERF-LC-MS, NB-4, NCI-H2286, SNU-61, T-47D, huH-1, KYSE-180, ST486, SW 1353, M-07e, KASUMI-1, YH-13, NCI-H28, GAMG, JeKo-1, GOS-3, SNU-324, PA-TU-8902, MFE-280, SNU-245, NALM-1, RERF-LC-Sq1, BICR 22, ZR-75-1, COR-L23, SW579, COR-L88, KM12, Hs 611.T, OUMS-23, RERF-LC-Ad1, NCI-H1385, SK-LMS-1, COLO-320, BL-70, GRANTA-519, MCAS, Panc 08.13, AM-38, KMS-11, SIG-M5, SNU-407, JHOS-2, OVCAR-4, Set-2, OV-90, MeWo, HEL, HT-29, MDA-MB-231, TOV-21G, NCI-H1355, KMS-27, NALM-6, KMS-26, Caov-4, KASUMI-2, UACC-62, U266B1, Hs 695T, HT55, BICR 31, TCC-PAN2, KMS-20, Hs 578T, RI-1, Hs 606.T, NCI-H1341, THP-1, BCP-1, Hs 737.T, SW1417, MOLT-4, Raji, ESS-1, MEL-JUSO, SH-10-TC, Hs 683, ME-1, EB2, PLC/PRF/5, NCI-H1339, A4/Fuk, SEM, HEC-265, IST-MES2, KE-97, NCI-H1437, COLO-704, NCI-H1915, TE-5, NCI-H2023, NCI-H82, T1-73, SNU-840, HuT 102, NCI-H1944, KYSE-520, Kasumi-6, 1321N1, Hs 742.T, IM95, PL45, CL-40, WM1799, KMM-1, SNU-449, JHUEM-1, KARPAS-620, Loucy, SNU-1079, Daudi, HCC-56, HSC-2, COR-L47, PA-TU-8988S, OAW28, COR-L311, L-363, Malme-3M, NOMO-1, Hs 870.T, SU-DHL-10, Hs 229.T, NCI-H810, KYSE-410, RPMI-8402, SNU-175, EBC-1, RVH-421, K029AX, PA-TU-8988T, LXF-289, OVSAHO, CAL-12T, Hs 940.T, MM1-S, SUP-HD1, LNCaP clone FGC, HSC-4, NU-DHL-1, NCI-H2228, BEN, CAL-78, Sq-1, NCI-H1793, SNU-C2A, MDA-MB-134-VI, COV318, KE-37, TYK-nu, MOTN-1, T98G, SW837, EB1, Becker, PE/CA-PJ34 (clone C12), Hs 616.T, NCI-H446, WM-88, CHP-126, Calu-1, SNU-283, NCI-H1573, SW 1271, SNU-16, JHOS-4, ACHN, Calu-3, KMRC-1, SW 1783, TE-11, TE-9, HuH-6, P31/FUJ, HT-1376, NCI-H520, 786-0, KNS-60, Caki-2, OVK18, PL-21, NCI-H2452, JURL-MK1, TEN, JHH-7, MDA-MB-157, Calu-6, RKN, NUGC-2, ONS-76, J82, OUMS-27, SNU-1196, Hs 739.T, RPMI-7951, NCI-H854, JHH-5, JVM-2, Hey-A8, 5637, KYSE-140, Capan-2, KYSE-150, HEC-1-B, BICR 16, HEL 92.1.7, MHH-NB-11, SNU-387, SK-OV-3, SK-MEL-28, IGROV1, ML-1, HLF-a, CHL-1, YKG1, A-204, OCI-M1, 8505C, JVM-3, NCI-H647, DB, COLO-800, PK-59, FaDu, HLF, OVMANA, EFO-27, PF-382, NCI-H747, LS123, SU-DHL-6, SJRH30, PANC-1, NCI-H2342, KM-H2, DND-41, HH, HuCCT1, F-36P, DMS 454, Hs 274.T, AU565, NCI-H1666, EN, RH-41, NCI-H1373, NCI-H838, SK-MEL-30, MOLM-6, DEL, NCI-H226, NCI-H1648, NCI-H661, 143B, Mino, C32, KMS-34, NCI-H1694, SK-ES-1, UACC-812, GDM-1, NCI-H23, Panc 02.03, CCF-STTG1, LOX IMVI, SJSA-1, MDST8, PK-1, NCI-H716, SU-DHL-4, MPP 89, MJ, COLO 829, PE/CA-PJ15, HD-MY-Z, BxPC-3, WM-793, COLO 668, T84, JHOM-1, PEER, LS411N, GMS-10, KMBC-2, RMG-I, KELLY, SNU-761, NALM-19, HEC-151, G-361, OVTOKO, A-498, SW 900, LCLC-103H, FTC-133, QGP-1, Reh, CMK-11-5, NU-DUL-1, BT-20, Hs 600.T, Hs 604.T, KATO III, SNU-410, NCI-H2126, SK-MEL-5, MDA-MB-468, AsPC-1, HUP-T3, KP-N-SI9s, L-428, SNU-1105, HUP-T4, 769-P, LMSU, NCI-H1869, NCO2, MOLM-16, CAL 27, HCC70, NCI-H1930, COV644, Hs 863.T, HCC-2279, D283 Med, Hs 944.T, HCC1599, MDA-MB-415, HCC2157, NCI-H1618, SNU-308, HCC1954, DMS 153, HPAF-II, T24, CJM, VM-CUB1, UM-UC-3, LAMA-84, NCI-H1734, JHH-2, VMRC-RCZ, MFE-319, MDA-MB-453, SNU-503, TOV-112D, B-CPAP, GSU, HCC-78, NCI-H2171, CAMA-1, HEC-108, HCC4006, CAL-85-1, NCI-H2122, COLO-699, NCI-H196, LUDLU-1, SW 780, RPMI 8226, LP-1, PC-14, HuTu 80, T.T, SW948, 22Rv1, HARA, NCI-H596, IPC-298, SCaBER, NCI-H1838, NB-1, Hs 934.T, Hs 895.T, DMS 114, KYSE-70, KP-3, KP4, DAN-G, NCI-H2009, OC 316, SCC-25, U-138 MG, RCC1ORGB, MFE-296, NCI-H1755, RERF-LC-KJ, 8305C, WSU-DLCL2, ES-2, MSTO-211H, SCC-15, ZR-75-30, PSN1, SNU-423, NCI-H2106, TE-1, UT-7, KMS-28BM, NCI-H2081, SK-MM-2, COLO 741, OC 314, HCC1395, MOLT-13, LN-18, Panc 10.05, PE/CA-PJ41 (clone D2), Hs 746T, CW-2, SKM-1, NUGC-3, TE-10, NCI-H358, NCI-H69, BFTC-909, HOS, BICR 18, NCI-H1395, OVKATE, Hs 698.T, EFM-19, COLO-783, MHH-CALL-4, ACC-MESO-1, NCI-H1436, KP-N-RT-BM-1, SK-MEL-31, NCI-H1105, CAL-51, YD-15, NCI-H2085, NCI-H2444, HCC1187, Hs 939.T, CAL-120, SCC-9, TUHR14TKB, KMRC-2, KG-1-C, ECC10, CGTH-W-1, NCI-H841, C2BBel, SUP-T11, RCH-ACV, CADO-ES1, JURKAT, 647-V, SK-MEL-2, MDA-MB-175-VII, MKN74, SNU-C4, LCLC-97TM1, SCC-4, BHY, IGR-37, KYO-1, Hs 281.T, TT, TUHR4TKB, HT-1080, NCI-H660, TE 441.T, LS1034, KNS-42, Panc 04.03, HCC1419, AZ-521, SNG-M, NCI-N87, G-292, clone A141B1, KPL-1, MDA-MB-361, CL-14, NCI-H2170, HuH-7, RD, NCI-H2066, IGR-1, TE-14, VCaP, BL-41, SNU-620, SK-MES-1, MEC-2, NCI-H1299, IGR-39, RT112/84, SF-295, DV-90, A2780, BICR 56, NCI-H510, NCI-H2141, YD-8, NCI-H2405, TF-1, MEC-1, CCK-81, NCI-H1048, Hs 822.T, NCI-H2052, K052, CAL-54, Hs 840.T, SW620, SK-CO-1, BT-474, CL-11, KNS-62, NCI-H1650, G-401, MOLT-16, SNU-398, COLO-680N, EM-2, Hs 294T, CAL-62, KMRC-3, A101D, KG-1, BT-549, HT115, A-375, SW-1710, WM-115, KLE, JHUEM-3, MKN7, CHP-212, HCC202, BC-3C, NCI-H1568, KMS-18, PE/CA-PJ49, COLO-849, SIMA, OCI-AML3, GSS, EC-GI-10, EFO-21, RCM-1, DMS 273, KU-19-19, RERF-GC-1B, SH-4, SK-MEL-3, RERF-LC-Ad2, M059K, JHOM-2B, MDA PCa 2b, Hs 852.T, RL95-2, Panc 03.27, SNU-216, Panc 02.13, CFPAC-1, SK-N-SH, OCI-AML2, LoVo, SBC-5, NCI-H1876, NCI-H441, SK-N-AS, COR-L24, HCC38, NCI-H1781, DOHH-2, NCI-H1563, U-251 MG, HPAC, JIMT-1, U-2 OS, A-673, TC-71, NCI-H650, NIH:OVCAR-3, CAS-1, JL-1, SK-MEL-1, MDA-MB-435S, Ishikawa (Heraklio) 02 ER-, TE 617.T, SU.86.86, RERF-LC-AI, TT2609-0O2, LS 180, YAPC, HDQ-P1, KNS-81, FU-OV-1, KP-2, DMS 53, SNU-1272, Detroit 562, 42-MG-BA, L3.3, COLO-679, NCI-H2087, NCI-H2030, GCT, NCI-H889, Caov-3, MDA-MB-436, NCI-H524, MKN1, KCL-22, Capan-1, CML-T1, H4, NCI-H727, Hs 343.T, MHH-ES-1, NMC-G1, HCC-1171, REC-1, Hs 618.T, A172, YD-10B, SW48, MUTZ-5, TE-6, JHH-1, HCT 116, TE-4, IA-LM, MG-63, NCI-H1975, TALL-1, HCC1806, HMCB, SCLC-21H, HCC1500, CL-34, Panc 05.04, SW403, TM-31, HCC1937, JMSU-1, DMS 79, SNB-19, NCI-H1836, Li-7, HCC827, 639-V, MOLM-13, SK-BR-3, IMR-32, TUHR1OTKB, OAW42, SK-N-MC, TGBC11TKB, NCI-H1581, EFM-192A, YMB-1, HCC2935, ECC12, HCC-33, DU 145, NCI-H146, SNU-1214, SNU-1077, 23132/87, HT-144, SNU-182, Hs 888.T, SNU-475, GCIY, Hs 729, JHOC-5, SW 1573, HEC-6, OCI-AML5, Hs 688(A).T, Hs 821.T, PCM6, RT-112, SK-N-DZ, SNU-478, SNU-119, HCC1143, NCI-H209, 8-MG-BA, COR-L105, COR-L95, SNU-46, COV504, CAL-148, SNU-05, DBTRG-05MG, BHT-101, WM-266-4, BFTC-905, KYSE-270, TE-8, SNU-213, and SH-SY5Y.

Test Compounds

Test compounds can be naturally occurring or synthetically produced. Proteins, polypeptides, peptides, polysaccharides, small molecules, and inorganic salts are examples of test compounds that can be screened using methods disclosed herein.

Any nucleic acid molecule that encodes these proteins can be introduced into a cell. The nucleic acid can be stably or transiently introduced into the cell. The exogenous nucleic acid can be in a construct or vector that comprises a promoter that is operably linked to the coding portion of the nucleic acid.

Cells(s) can be grown on an appropriate substrate, such as a multi-well plate, a tissue culture dish, a flask, etc.; see Examples 1 and 2, below.

Screening Methods

In some embodiments, methods of identifying a sweet taste modulator comprise contacting a cell with a test compound and measuring sweet taste receptor activity. A change in sweet taste receptor activity by the test compound indicates modulation of the sweet taste receptor by the test compound, thereby identifying the test compound as a sweet taste modulator.

Sweet taste receptor activity can be measured quantitatively or qualitatively by any means standard that is in the art, including assays for G protein-coupled receptor activity, changes in the level of a second messenger in the cell, formation of inositol triphosphate (IP₃) through phospholipase C-mediated hydrolysis of phosphatidylinositol, changes in cytoplasmic calcium ion levels, β-arrestin recruitment, and the like. Examples of such assays are provided in the specific examples, below.

Binding activity can also be used to measure taste receptor activity, for example, via competitive binding assay or surface plasmon resonance. Receptor internalization and/or receptor desensitization can also be measured, as is known in the art. Receptor-dependent activation of gene transcription can also be measured to assess taste receptor activity. The amount of transcription may be measured by using any method known to those of skill in the art. For example, mRNA expression of the protein of interest may be detected using PCR techniques, microarray or Northern blot. The amount of a polypeptide produced by an mRNA can be determined by methods that are standard in the art for quantitating proteins in a cell, such as Western blotting, ELISA, ELISPOT, immunoprecipitation, immunofluorescence (e.g., FACS), immunohistochemistry, immunocytochemistry, etc., as well as any other method now known or later developed for quantitating protein in or produced by a cell.

Physical changes to a cell can also be measured, for example, by microscopically assessing size, shape, density or any other physical change mediated by taste receptor activation. Flow cytometry can also be utilized to assess physical changes and/or determine the presence or absence of cellular markers.

In some embodiments, sweet taste receptor activity is measured by detecting the level of an intracellular second messenger in the cell (e.g., cAMP, cAMP, cGMP, NO, CO, H2S cGMP, DAG, IP3). In some embodiments, sweet taste receptor activity is measured by detecting the level of intracellular calcium. In some embodiments, the sweet taste receptor activity is binding activity.

In any of these embodiments, the cell can modified to overexpress the sweet taste receptor and or Gα15, as described below.

Those skilled in the art will appreciate that there are numerous variations and permutations of the above described embodiments that fall within the scope of the appended claims.

EXAMPLE 1 Generation of JUMP-IN™ T-REX™ U2-OS Cell Lines and Targetable JUMP-IN™ U2-OS Cell Lines

Generation of JUMP-IN™ T-REX™ U2-OS Cell Lines. JUMP-IN™ U2-OS cells were transfected with the pLenti TR puro vector using LIPOFECTAMINE™ LTX reagent (Life Technologies # 15338-100). Transfected cells were selected with 1 μg of puromycin (Life Technologies # A1113803), and the non-transfected parental cells were used as an antibiotic-selection negative control.

The selected clones were expanded and transiently transfected with the pJTI™-R4 EXP CMV TO GFP pA vector (Life Technologies). The cells were then tested for tet-repression and tet-inducibility with or without 1 μg of doxycycline, and GFP expression levels were quantified with a Safire II microplate reader.

Eighty four clones survived after the FACS sort. Each clone was plated in triplicate in 96-well plates. One well was used for maintenance, and the other two wells were used for transient transfection with a pJTI™ R4 EXP CMV-TO pA expression vector (Life Technologies) encoding green fluorescent protein (GFP) for 24 hours. Cells were then treated with or without 1 μg doxcycline for 24 hours, followed by GFP quantification. Response ratio was calculated as

$\frac{\left( {{reading}\mspace{14mu} {of}\mspace{14mu} {positive}\mspace{14mu} {doxycycline}\mspace{14mu} {well}} \right)}{\left. {{reading}\mspace{14mu} {of}\mspace{14mu} {negative}\mspace{14mu} {doxycycline}\mspace{14mu} {well}} \right)}$

Twelve JUMP-IN™ T-REX™ U2-OS clones with a response ratio over 2 were further expanded. Five of these clones were co-transfected with the R4 integrase construct and p.JTI™-R4 EXP CMV-TO GFP vector (providing inducible GFP expression). Cells co-transfected with a CMV-GFP construct (providing constitutive GFP expression) and the R4 integrase construct were used as a positive control for transfection and antibiotic selection. After selection with 5 μg/mL of blasticidin for 28 days, cells that were retargeted with the pJTI™-R4 EXP CMV-TO GFP vector were induced with 1 μg/mL of doxycycline for 20 hours and then analyzed by FACS.

Generation of Targetable JUMP-IN™ U2-OS Cell Lines. Targetable JUMP-IN™ cell lines were engineered using the JUMP-IN™ TI™ Platform Kit (Life Technologies). The pJI™ PhiC31 Int vector was co-transfected into U2-OS host cells using a pJTI™/Blasticidin targeting vector, which contains ATT sites which are complementary to pseudo-ATT sites in the mammalian host genome. Homologous recombination between the ATT sites of the R4 vector and the host genome are facilitated by the phiC31 integrase. The pJI™ targeting vector contains the hygromycin resistance gene, which permits selection of cells containing stable genomic integrations. These cells were then cloned by flow cytometry and tested to determine the number of R4 sites present in the host cell genome by Southern blot analysis.

Clones with validated single R4 integration sites were validated for retargeting by transfection with the pJTI™-R4 EXP CMV-TO-EmGFP-pA and JTI™ R4 integrase vectors, followed by antibiotic selection for 4 weeks. Clones were plated at 60-80% confluency in a 6-well dish in growth medium without antibiotics (McCoy's 5A plus 10% dialyzed fetal bovine serum, HEPES, non-essential amino acids, and sodium pyruvate) and transfected with a 1:1 ratio of pJTI™-R4 EXP CMV-TO-EmGFP-pA and JTI™ R4 integrase vectors (2.5 μg DNA total) with LIPOFECTAMINE™ LTX (6.25 μl) and PLUS™ Reagent (2.5 μl). The CMV-GFP positive control was introduced into the cells in parallel. Following overnight incubation, the cells were selected for 28 days with 5 μg/mL of blasticidin in growth medium (McCoy's 5A plus 10% dialyzed FBS, HEPES, non-essential amino acids, sodium pyruvate, and penicillin/streptomycin). After 28 days of antibiotic selection, the selected pools were analyzed by FACS.

Five clones were chosen to test retargeting efficiency and inducible potential. All clones were retargeted successfully. The two clones which had the best retargeting efficiency were further tested for inducible potential by doxycycline. One clone demonstrated ˜88% GFP cells in the GFP retargeted control cells and responded very well to doxycycline induction with >99% GFP positive population in the induced culture and only ˜4% of GFP positive cells in the non-induced culture. The other clone also responded well to doxycyline induction with >99% GFP positive cells in the induced culture and only ˜8% of GFP positive cells in the non-induced culture; with 64% of cells were GFP positive in the GFP retargeted control cells.

Cell Culture Medium. The table below lists the components of the medium used for JUMP-IN™ T-REX™ U2-OS cell culture (Life Technologies):

Cell Culture Medium and Reagents Amount Cat. No. McCoy's 5A Medium (modified) (1x), 500 mL 16600-082 Liquid Fetal bovine serum (FBS), dialyzed 100 mL 26400-036 Penicillin/Streptomycin, 10,000 U/10,000 pg 100 mL 15140-122 Hygromycin B (50 mg/mL) 20 mL 10687-010 Puromycin Dihydrochloride (10 mg/mL) 1 mL A11138-03 Phosphate-buffered saline without calcium or 1000 mL 14190-136 magnesium

EXAMPLE 2 Methods

Cell culture and compound treatment. U2-OS cells were grown in McCoy media supplemented with 10% fetal bovine serum. Cells were seeded at density of 5,000 cells/well on PDL-coated 384-well plates.

BacMam transfection. U2-OS cells were transiently transfected with 0.5 -12 μl of BacMam encoding G protein, gustducin (Gα15, Ga16gust44) per 40,000 cells during 24 h or 48 h. Transfected cells were seeded at density of 5,000 cells/well on PDL-coated 384-well plates.

Functional expression. JUMP-IN™ T-REX™ U2-OS cells were transfected with the pJTI™R4EXP-CMV-TO-T1R3/T1R2/Gα15-pA and JTI™R4 Integrase vectors (Life Technology). Doxycycline-induced expression of T1R3/T1R2/Gα15 was detected by RT-PCR.

pERK½ assay. For pERK½ studies, growth media was replaced with media without serum for 2 h. U2-OS cells were treated with 100 mM Sweet compounds with or without of U73122 (10 μM) and with or without lactisole (20 mM). Control groups of cells also received DMSO (0.1%) in medium. The fixed cells were stained with rabbit anti-phospho-p44/p42 MAPK (pERK½) antibodies from Cell Signaling Technologies, then with secondary Alexa 488-conjugated antibodies (Life Technology) and Hoechst 33342. Images were acquired on an ImageXpressMicro (MDC) and analyzed with MWCS Module.

Calcium assay. Calcium traces in Fluo-4AM loaded cells were recorded over time in FLIPR^(Tetra) (MDC) with λ_(EX)=470-495 nm and λ_(EM)=515-575 nm. The peak increase in fluorescence over baseline was determined and is expressed as relative fluorescence units (RFU).

Transfluor model. Transfluor U2-OS cells stably expressing β-arrestin-GFP were transiently transfected with taste G protein, gustducin (Gα15 or Gα16gust44) (Life Technologies) or were transiently transfected with sweet receptor T1R2/T1R3 and taste G protein. Treatment of cells with sweet-tasting compounds induced formation of fluorescent vesicles. Sweet receptor internalization was quantitated via the Transfluor Application Module of the MetaXpress Software (MDC).

EXAMPLE 3 Phosphorylation of ERK½

The phosphorylation of ERK½ is a common downstream event following GPCR activation via the PLCβ2-IP3 pathway. Growth media was replaced with Dulbecco's phosphate buffered saline (DPBS) without serum for 2 h. U2-OS cells were treated with 100 mM D-glucose with or without of 20 mM of lactisole for 5, 15 and 30 minutes and then fixed and stained with anti-pERK½ and Hoechst 33342 before imaging. pERK½ expression was quantitated using Molecular Devices' Multiwaves Cell Scoring analysis algorithm. For each data point, average of Mean Cell Integrated intensity and standard deviation are calculated from quadruplicate data points. D-glucose induced pERK½ activation in a time-dependent manner in parental U2-OS (FIG. 4, left) and in U2-OS cells overexpressing Gα15 (FIG. 5, left). Importantly, pERK½ activity was blocked by the sweet receptor antagonist, lactisole (FIG. 4, right) or the PLCβ2 inhibitor U73122 (FIG. 5, right).

EXAMPLE 4 Overexpression of Gα15

Overexpression of Gα15 in a U2-OS cell significantly increases signaling in response to a sweet taste modulator. Sucralose induced Ca⁽²⁺⁾ response in U2-OS cell line transfected with Gα15 in concentration-dependent manner (FIG. 2). We observed the highest Ca⁽²⁺⁾ response in U2-OS cells transfected with 12 gl/40,000 cells (FIG. 2). Transfection of U2-OS cells with 6 μl/40,000 cells significantly increased Ca⁽²⁺⁾ response upon treatment with natural sugars, such as glucose, fructose, and sucrose (FIG. 3). Furthermore, overexpression of Gα15 increased pERK½ activity upon treatment with glucose, or maltose in U2-OS cells (FIG. 6). Importantly pERK½ activation was blocked by PLCβ2 inhibitor U73122 (FIG. 7).

EXAMPLE 5 Ca⁽²⁺⁾ Response

We have found that both monosaccharaides and disaccharides induced a concentration-dependent Ca⁽²⁺⁾ response in JUMP-IN™ T-REX™ U2-OS cell line with doxycycline-inducible expression of sweet taste receptor T1R2/T1R3/Gα15 (FIG. 1A). T1R2/T1R3-mediated Ca⁽²⁺⁾ release was inhibited by lactisole (FIG. 1B). Moreover, U73122 abolished Ca⁽²⁺⁾ release upon treatment with D-glucose and D-fructose and had no effect on Ca⁽²⁺⁾ response after stimulation with sucrose and sucralose (FIG. 1B).

Taken together, these data demonstrate a relationship between the molecular structure of sugars and the T1R2/T1R3 sweet taste receptor.

EXAMPLE 6 Artificial Sweeteners

Some striking observations were made with the artificial sweeteners, Ace-K, aspartame, and saccharine. Ace-K and aspartame did not induce T 1R2/T 1R3-mediated Ca⁽²⁺⁾ response in JUMP-IN™ T-REX™ U2-OS cells (FIG. 1A). T1R2/T1R3-independent Ca⁽²⁺⁾ release occurred upon treatment with saccharine (FIG. 1A), indicating that artificial sweeteners Ace-K, aspartame, and saccharine target common receptor which was yet to be identified.

EXAMPLE 7 Effects of Sweet-Tasting Compounds on Recruiting β-Arrestin to Sweet Taste Receptor

Multiple lines of evidence suggested that GPCR ligands selectively activate β-arrestin signaling pathway. To assess the effects of sweet-tasting compounds on recruiting β-arrestin to sweet taste receptor, we used a Transfluor Model (MDC), which is highly sensitive to 13-arrestin redistribution. Treatment of cells with sweet-tasting compounds induced formation of fluorescent vesicles, indicating internalization of T1R2/T1R3-β-arrestin complexes into endosomes (FIG. 8A). Sweet receptor internalization was quantitated via the Transfluor Application Module of the MetaXpress Software (MDC). D-glucose, D-fructose, and Ace-K slightly increased sweet receptor internalization, whereas sucrose, sucralose, and aspartame had strong effect on T1R2/T1R3 internalization (FIG. 8B). In contrast, even minimal internalization did not occur upon treatment with saccharine (FIG. 8B). 

1. A method of identifying a sweet taste modulator comprising: a) contacting a U2-OS cell with a test compound; and b) measuring sweet taste receptor activity, wherein a change in sweet taste receptor activity by the test compound indicates modulation of the sweet taste receptor by the test compound, thereby identifying the test compound as a sweet taste modulator.
 2. The method of claim 1, wherein the U2-OS cell is modified to overexpress the sweet taste receptor.
 3. The method of claim 1, wherein the sweet taste receptor activity is measured by detecting the level of an intracellular second messenger in the U2-OS cell.
 4. The method of claim 3, wherein the second messenger is cAMP, cGMP, NO, CO, or H2S.
 5. The method of claim 3, wherein the second messenger is DAG or IP3.
 6. The method of claim 1, wherein sweet taste receptor activity is measured by detecting the level of intracellular calcium in the cell.
 7. The method of claim 1, wherein the sweet taste receptor activity is binding activity.
 8. The method of claim 7, wherein a change in binding activity is detected by a competitive binding assay.
 9. The method of claim 7, wherein a change in binding activity is detected by surface plasmon resonance.
 10. The method of claim 1, wherein sweet taste receptor activity is measured by detecting phosphorylation of ERK½.
 11. The method of claim 1, wherein sweet taste receptor activity is measured by detecting internalization of the receptor.
 12. The method of any of claims 1-11, wherein the U2-OS cell further comprises an exogenous nucleic acid encoding Gα15, Gα16, Gα16gust25, Gα15gust25, Gα16gust44, Gα15gust44, or Gα15-i/3-5.
 13. The method of any of claims 1-12, wherein the U2-OS cell comprises a Tet-repressor protein expression construct.
 14. An isolated U2-OS cell, comprising an exogenous nucleic acid encoding Gα15, Gα16, Gα16gust25, Gα15gust25, Gα16gust44, Gα15gust44, or Gα15-i/3-5.
 15. An isolated U2-OS cell, comprising one or more exogenous nucleic acids encoding a sweet taste receptor.
 16. An isolated U2-OS cell, comprising an exogenous nucleic acid encoding a detectable label.
 17. The isolated U2-OS cell of claim 16, wherein the detectable label is β-arrestin GFP.
 18. The isolated U2-OS cell of claim 14 or 15, further comprising an exogenous nucleic acid encoding a detectable label.
 19. The isolated U2-OS cell of any of claims 14-16, further comprising an exogenous nucleic acid encoding Gα15, Gα16, Gα16gust25, Gα15gust25, Gα16gust44, Gα15gust44, or Gα15-i/3-5. 